Wire bonding is one of the key manufacturing processes in electronic packaging in which a die comprising integrated circuits, is electrically connected to a substrate to which it has been attached by fine wires of conductive material, such as aluminum, copper or gold wires.
A vast majority of wire-bonded interconnections are made with thermosonic bonding. Thermosonic bonding is a combination of ultrasonic and thermocompression welding that optimizes the best qualities of each for microelectronic usage. Thermocompression welding per se usually requires interfacial temperatures of at least 300° C. Such temperatures may damage some die-attach plastics, packaging materials, laminates, as well as some heat-sensitive dice.
However, utilizing thermosonic bonding, the interface temperature can be much lower, typically between 125° C. and 220° C., which avoids such problems with heat damage. The ultrasonic energy helps disburse contaminants during the early part of the bonding cycle and the thermal energy helps to mature the weld through diffusion of energy to promote metallic diffusion and hence intermetallic growth at the interface of the bond sites. This combination of ultrasonic and thermocompression welding also allows the ultrasonic energy to be kept small enough to minimize cratering damage to the semiconductor die. Thus, the connectivity and strength of the thermosonic bonds substantially depend on thermal diffusivity of energy from a heating source to the bond sites, and hence the effectiveness of thermal transfer to the bond sites is important.
In conventional electronic devices, a substrate is usually attached with a single die to which thermal energy can be directly transferred from a heating source underneath the substrate to a bond site on the die by conduction. As the combined thickness of the single die and substrate is relatively thin, the rate of thermal diffusion through the substrate and then through the die is quite fast.
However, with improvement in technology, there are now semiconductor devices wherein a die is attached onto another die to increase functionality of the device. The die that is stacked on top of another die may overhang from the bottom die, and the overhang may be more than a few millimeters. Furthermore, a substrate may be mounted with multiple layers of dice. FIG. 1 is an illustration of a multi-layer stacked dice assembly 10 of the prior art. The multi-layer stacked dice assembly 10 comprises a plurality of individual dice 12 orthogonally arranged with respect to one another and mounted on top of a substrate 14. The stacked dice 12 are electrically connected to the substrate 14 by a plurality of bonded wires 16.
It will be convenient to hereinafter illustrate the thermal diffusivity within the multi-layer stacked dice assembly 10 by reference to a single die stacked onto a second die on a substrate, whereby thermal diffusion through multiple layers of stacked dice can be demonstrated.
FIG. 2 is an isometric view of a stacked dice assembly 20 being heated with a conventional heat transfer system of the prior art. The stacked dice assembly 20 comprises a first die 22 mounted on a second die 24, which is in turn mounted on a substrate 26. Layers of adhesive 28 are disposed between the first die 22 and the second die 24, and between the second die 24 and the substrate 26 respectively. The first die 22 comprises a plurality of bond pads 30 on its top surface. The stacked dice assembly 20 in FIG. 2 is shown partly bonded with bonding wires 32 interconnecting the bond pads 30 to leads 34 on the substrate 26.
A heating source 36, such as a hot plate, is introduced beneath the substrate 26 for providing thermal energy during the thermosonic bonding process. As the thermal energy is received by the substrate 26, it is diffused upwards from the substrate 26 through the first layer of adhesive 28 to the second die 24, and then through another layer of adhesive 28 to the first die 22, and eventually to the bond pads 30 on the first die 22.
Thermal diffusion is omni-directional and thermal energy diffuses from points of higher temperature towards the boundaries of components where the temperatures are lower. Due to such temperature differences, thermal energy transfer by conduction occurs within components of the stacked dice assembly 20. Internal arrows 38 indicate the flow of thermal energy conducted through the components.
However, in normal operating environments, the stacked dice assembly 20 is surrounded by ambient air of lower temperature. The thermal energy is able to diffuse into and be lost through the ambient air. Besides this, there is also energy being lost by advection through the bulk motion of air. Air flow is induced by buoyancy forces which arise from density differences caused by temperature variation in the air near the surfaces of the stacked dice assembly 20 and the ambient atmosphere. This causes natural convection heat lost to the environment. In FIG. 2, external arrows 40 indicate the natural convection heat lost to the environment.
Such convection heat loss occurs over the surfaces of the stacked dice assembly 20. The amount of heat lost through natural convection therefore increases when larger surface areas are exposed to the atmosphere. An overhanging die, and especially multiple layers of overhanging stacked dice 12 packed in the manner shown in FIG. 1, have large surface areas exposed to the ambient atmosphere. Thus, stacked dice are more susceptible to natural convection heat loss than single dice.
Moreover, the orthogonal arrangements of adjacent dice 12 in the stacked dice assembly 10 reduce the die-to-die contact surfaces needed for thermal diffusion through the components. Therefore, the thermal diffusivity of the stacked dice assembly 10 through conduction is adversely affected.
In addition, the semiconductor dice, which are made of silicon, are inherently poor thermal conductors and have high thermal resistance. The thermal resistance effect is further compounded by the layers of adhesive disposed at the interfaces between the die and substrate, and the interfaces between adjacent dice respectively. The adhesives usually comprise polymeric materials, which are inherently poor thermal conductors and have high thermal resistance. Hence, the inherent material properties of the multi-layered silicon dice and the polymeric adhesive detrimentally affect the thermal diffusivity of the stacked dice assembly 10.
In cases where die bending needs to be minimized, and/or when a higher loop height for the bottom die is required, thicker dice are used in the stacked dice assembly. An increase in the thickness of a die, however, will substantially increase its thermal resistance, and hence decrease its thermal diffusivity. Similarly, the thicker the adhesive between the dice, the lower is the thermal diffusivity of the stacked dice assembly 10.
Furthermore, the rate of heat loss depends on the temperature differences between the stacked dice assembly 10 and the ambient atmosphere. A temperature gradient between the stacked dice assembly 10 and the ambient atmosphere will generally vary according to environmental temperature changes. Therefore, the temperature on the bond pads of the stacked dice assembly 10 is unpredictable when the environmental temperature changes, causing difficulty in controlling the temperature at the bond pads.
As such, the conventional heat transfer system is unable to consistently provide a constant bonding temperature to the bond pads 30, thereby reducing the effectiveness of the thermosonic bonding being performed. The connectivity and strength of the bonds may be affected.
Accordingly, it would be desirable to alleviate the aforesaid disadvantages of the prior art by providing a simple and cost effective apparatus for transferring thermal energy efficiently to the stacked dice during wire bonding in order to achieve a consistent bonding temperature at the bond pads 30.